Using minecraft's Towny mod as a base for this discussion in it you can buy chunks(16mx16m) to build a town. But you need to use your in-game money made from resource mining to buy the first chunk which is $75,000. Each additional chunk is $500.
In Outerra since we are basically GIVING everyone the first section for "free" maybe they need to pay off that "free" plot before they can expand. Prove that they have the skills to invest more land.
Maybe the mothership is loaded with real estate lawyers!
I think a strict limit of say 10 sections for a single user then when multi-player is enabled. A group can combine and the chunks can add up but not in a linear way. 10 for a single.. 15 for two people, 18 for three, 20 for four, and two additional blocks for each person thereafter. That way a city can be huge but it will have to have a large amount of people. You would then need to block certain abilities from smaller groups. Airports, distribution hub's and mega-highway machines could be reserved for cites with populations of 100, 250?
Perhaps a founder system. Where the city only "belongs" to founding members so people can keep their 10 slot plot and work for the city.
Then since web integration is so easy each City can get a forums to post plans, organize leaders and events. A community could literally be that.